A typical fuel pumping unit for an aero-engine comprises a low pressure (LP) pump operable to draw fuel from a fuel tank, and supplying the fuel at boosted pressure to the inlet of a high pressure (HP) pump. The inter-stage flow between LP and HP pumps is typically used to cool engine lubrication oil in a fuel/oil heat exchanger.
Commonly, the LP pump comprises a centrifugal impeller pump whilst the HP pump comprises a positive displacement pump in the form of a twin pinion gear pump. The pumps are generally driven from a shaft on the engine accessory gearbox and therefore have a fixed speed relationship with engine running speed.
At sea level, aviation fuel dissolves air. When the partial pressure of the fuel tanks is reduced, due to increase in altitude, this air is released from the fuel. The resultant air/fuel mixture produces a multi-phase flow. Multi-phase flow can also occur at sea level, albeit to a lesser extent, as a result of pressure losses along the pipes.
In order to operate correctly and reliably, the HP pump requires fuel in a homogeneous state and not an air/fuel mixture. Increasing the inlet pressure to the HP pump forces, any undissolved air back into solution. The required pressure rise is normally provided by the operation of the aircraft wing tank pumps. The fuel pumping unit, however, should be able to operate correctly even when the wing pumps are not functioning. In this case, the required HP pump inlet pressure rise is provided solely by the LP pump.